Strip transmission line switch



Nov. 19, 1957 w. M. FURLOW, JR., ETAL STRIP-TRANSMISSION LINE Filed June 7, 1955 SWITCH INVENTORS Lew s d. War-re Jr. l v/l/lam M. Fur/ow Jr:

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Unite States Patent Worrell, Jr., Hyattsville, Md., assignors to Melpar, Inc.,

Alexandria, Va., a corporation of New York Application June 7, 1955, Serial No. 513,874

6 Claims. (Cl. 333-97) The present invention relates generally to wide-band coupling networks of the long line type, and more particularly to wide-band coupling networks capable of utiliza tion for selectively making and breaking a circuit between two or more transmission line sections.

Waveguide and transmission line switches commonly find utility in systems for selectively connecting and disconnecting radio receivers and/or transmitters, and the like, from sources and loads, such as antennae or indicators. Zones of contact between elements of such switches generally introduce losses and reflections, especially in the microwave range of frequencies, and are conducive to the generation of noise due to relative motion of the contacts of the switches. Such relative motion may be introduced in microwave switches in response to mechanical vibration, and other variable physical eflects external of the switch. Where the switch is required to make and break a microwave circuit periodically and at a high periodicity, as in microwave commutators, wiping action between relatively movable elements of the switch introduces contact noise, and the discontinuity of microwave circuitry at the switch introduces losses and reflection of wave energy. Reflection is of paramount interest in microwave circuits employed for the transmission of high power, since reflection increases standing wave ratio, and thereby the possibility of transmission line breakdowns. In lower power receiver circuits reflection and noise increase implies decrease of reception quality and sensitivity.

It is, accordingly, desirable to provide microwave switches, when employed as elements of microwave commutators, which do not include contacts. Contacts may be avoided by providing a gap or gaps in microwave switches, and associating the gaps with microwave circuitry which cause the gaps to appear as continuous or closed elements by decreasing their apparent impedances to low values.

Where two transmission line elements are to be joined with provision for relative axial or rotary motion of the elements, coaxial transmission lines of circular cross section may be employed. The use of such lines is not readily feasible in microwave commutators wherein one movable element must travel past a plurality of stationary elements in sequence. In particular, when the movable element (rotor) may rotate about an axis, and must come into electrical coupling relation with circumferentially arranged stator elements of a commutator, in sequence, we employ'flat-strip transmission line elements. It is a feature of the present invention to provide a flatstrip transmission line microwave switch or commutator.

Briefly describing a specific embodiment of the invention, and keeping in mind that the structural features described as applying to the rotor may be interchanged with those applying to the stator, we employ as a stator a conductor of rectangular transverse cross section having a slot extending endwise into the conductor a distance of an odd number of quarter wavelengths at the center frequency of a band of operating frequencies. This con- Patented Nov. 19, 1957 ductor is the center conductor of a flat strip transmission line. The rotor element is of rectangular transverse cross section, and includes an element which extendswithin the stator slot, but not quite in contacting relation to the latter, and is electrically the same odd number of quarter wavelengths as the stator slot at the center frequencies of the band of operating frequencies. The height of the rotor element is slightly less than the height of the slot in the stator, so that two adjacent surfaces taken from the two elements represents a two conductor transmission line, of predeterminable characteristic impedance Z1, and of an odd number of quarter wavelengths. This line is open at its receiving end, and therefore reflects a low impedance at its sending end. The sending end terminates in a gap between rotor and stator, which is electrically closed, because of its low impedance, while physically it is open.

The structure described hereinbefore provides a noncontacting transmission line switch or commutator pair, but possesses the defect that the physical gap between rotor and stator presents a low impedance for only a very narrow band of frequencies. To broaden the band we provide an endwise slot in the rotor element, which may be filled with dielectric material, of the low loss type, or may be left open and empty. The dielectric material, when used, provides support for the walls of the slot. The height of this slot is great relative to the spacing between adjacent surfaces of the rotor and stator, and the length of the slot is the same odd number of quarter wavelengths as the stator slot at the center frequency of the operating band. The two parallel surfaces defining the slot establish a two wire transmission line, of an odd number of quarter wavelengths, and characteristic impedance Z2, shorted at one end and open at the other. The value of Z2 will be considerably higher than the value of Z1, and the two lines of respective characteristic impedances Z1 and Z2 are electrically in series, the high impedance line being shorted at its receiving end. The two lines together then being of one or more half wavelengths, reflect a low impedance at the gap between the rotor and stator. The mismatch between the quarter wave sections, however, widens the bandwidth over which the low impedance is reflected at the gap, and in fact permits the switch to be employed for wide band operation.

It is, accordingly, a broad object of the present invention to provide a microwave switch employing flat strip elements, and containing no physically contacting elements common to relatively movable elements of the switch.

Another object of the present invention is to provide a non-contacting flat-strip microwave switch for selectively coupling microwave energy between transmission lines.

Still another object of the present invention is to provide a non-contacting flat-strip switch for coupling at will microwave energy between two transmission lines, and operative over a wide band of frequencies.

A further object of the present invention is to provide a non-contacting flat strip transmission line'switch for microwave energy, which employs an end-slotted stationary electrode and a movable electrode rotatable through the slot of the stationary electrode, and wherein the dimensions of the movable electrode are such with respect to the dimensions of the slot as to define a transmission line having an electrical length equal to one or more quarter wave lengths at the operating frequency.

Yet another object of the present invention is to provide a non-contacting, fiat strip transmission line switch employing an end-slotted stationary electrode and a rotatable electrode selectively positionable in the slot in the stationary electrode, wherein the relative dimensions of the rotatable electrode and the slot are such as to define a transmission line having an electrical length equal to one or more quarter wave lengths at the operating frequency and wherein the rotatable electrode is provided with an end slot filled with dielectric material, and wherein the characteristic impedances of the slot in the movable electrode of the transmission line formed by the movable electrode and the slot in the stationary electrode, and of the transmission line in which the switch is connected, are suitably related to provide wide band operation.

, The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of one specific embodiment thereof, especially when taken in conjunction with the accompanying drawing, wherein:

The single figure of the drawing is an isometric view, partly in cross section, of a single set of contacts of the fiat line microwave switch of the present invention.

Referring now more particularly to the accompanying drawing, there is illustrated a flat strip transmission line switch, generally designated by the reference numeral 1, and comprising two flat metallic ground planes 2 lying in spaced parallel planes. Positioned equidistant between the ground planes 2 is an elongated central conductor 3, having a rectangular transverse cross section, the larger dimension of which is parallel to the ground planes 2. The ground planes 2 in conjunction with conductor 3 provide a length of transmission line having a characteristic impedance Z0, and capable of transmitting energy at V. H. F. and U. H. F. frequencies. We desire it to be understood that a single ground plane may be employed, if desired, and that if two ground planes are employed the central conductor need not be equidistant therebetween, although that is the usual configuration.

The conductor 3 comprises two coupled elements or electrodes 3a and 3b. The left hand element or electrode 3a of the central conductor 3, as seen in the accompanying drawing, is provided with a longitudinallyextending end-slot 4 the upper and lower boundaries of which are fiat plates 5 and 6, respectively, which are continuations of the center conductor 3 and lie in planes parallel to the conductors 2. The flat plates 5 and 6 taken together with the slot 4 therebetween constitute the first electrode 3a of the switch 1. The electrical length of the slot 4 is preferably one quarter wave length at the center frequency of the operating frequency band of the transmission line.

The element 3a may be selectively coupled with a second element 3b, positioned equidistant between the metallic conductors 2, and axially alignable with the conductor 3a to provide an electrically continuous conductor. The conductor 315 has a rectangular transverse cross section, identical with that of element 3a, and electrically provides a smooth continuation thereof. The element 3b comprises a main body 8 of the same cross-sectional dimensions as the central conductor 3 and an electrode 9, in the form of an end extension of said element 3b, having a smaller height than does the main body 8 of the conductor 3, and of identical width. When the switch is closed, as by rotation of the element 3b into alignment with the element 3a, the electrode 9 extends from the left end of the main body 8 into the slot 4 in the element 3a, being positioned symmetrically between the flat plates 5 and 6 of the element 3a. No physical contact between element 3a and element 3b subsists. The vertical dimen sion of the electrode 9 is such as to define a space 10 between the upper surface of the electrode 9 and the lower surface of plate 5, and a similar space 10 between the lower surface of electrode 9 and the upper surface of plate 6. The under surface of plate 5 in conjunction with the upper surface of electrode 9, and the under surface of electrode 9 in conjunction with the upper surface of the plate 6, define a pair of transmission lines, each of characteristic impedance Z1 and of electrical length equal preferably to one quarter of a wave length at the mean operatingfrequency. The transmission lines thus formed are each open at their receiving ends, thereby reflecting low impedance at their transmitting ends, and hence in the gaps 10 between the lines 3!: and 3b. Energy at frequency may be transmitted from the conductor 3b to conductor 3a or vice versa, with substantially no reflection or loss due to the lack of contact therebetween.

The electrode 9 is provided with an end slot of rectangular cross section extending longitudinally inwardly from the end of the electrode 9, and bounded by a pair of parallel strips 11 and 12, This slot is filled with a block of dielectric material 13, having low loss at U. H. F. frequencies. The strips 11 and 12 define a transmission line, open circuited at one end, having a short circuit at another end, 14, and having a characteristic impedance Z2 by reason of the spacing of strips 11 and 12, and the presence of dielectric material 13 therebetween, such that Z2 Z1.

The electrical length of path between gaps 10 and the short circuit at 14 is so that the short circuit at 14 transforms to short circuits at the open end of line 5, 6. The mismatch which occurs in passing from one of the quarter wave lines to the other, and the relative values of characteristic impedances involved, can be shown to establish a relatively wide band of frequencies for which a low or negligible impedance appears across the gaps 10. It can be shown that the band width of wave energy transferable between the elements 3a and 3b may be maximized if Z1 Z0 and Z2 Z1. (See Montgomery: Principles of Microwave Circuits, Rad. Lab. vol. 8, pp. 196 and 197.)

The coupling between line elements 3a and 3b is maintained so long as the line elements 3a and 3b are in alignment. Coupling may be reduced or eliminated by moving one element out of alignment with the other, as by rotating element 3b about an axis 15, in any convenient fashion.

While we have described and illustrated one specific embodiment of our invention, it will be clear that variations of the general arrangement and of the details of construction which are specifically illustrated and described may be resorted to without departing from the true spirit and scope of the invention.

What We claim is:

l. A fiat strip transmission line switch for switching energy in a frequency band centered about a predetermined frequency, said switch comprising two flat metallic conductors lying in parallel spaced planes, at least two switch electrodes lying in a common plane between and parallel to said two metallic conductors, one of said switch electrodes having an end-slot in one end thereof, said slot having an electrical length equal to one quarter wave length at said predetermined frequency, the other of said switch electrodes having an end element, said end element having an electrical length equal to one quarter wave length at said predetermined frequency, an endslot in said end of said other switch electrode, said slot beingfilled with a dielectric material and having an electrical length equal to one quarter wavelength at said predetermined frequency, said slot having a characteristic impedance Z2, means for selectively positioning saidend of said other switch electrode in said slot of said one electrode, the relative dimensions of said slot and said end element being such as to define a transmission line having a characteristic impedance Z1, the characteristic impedances being such that Z1 Zo and Zz Z wherein Z0 is the characteristic impedance of said transmission line.

2. A flat strip transmission line switch comprising two fiat metallic conductors lying in spaced parallel planes, at least two switch electrodes lying in a common plane between and parallel to said two metallic conductors, a slot in one end of one of said switch electrodes providing two inner surfaces parallel to said metallic conductors, one end portion of the other of said switch electrodes having a recess formed in each of its surfaces parallel to said metallic conductors to form an end portion having a reduced uniform cross section, said last named end portion having a slot providing interior surfaces parallel to said metallic conductors, said slot being filled with a dielectric material to provide a slot having an electrical length equal to one quarter wave length at the operating frequency and a characteristic impedance Z2, means for at will positioning said end portion in said slot in said one switch electrode, the relative dimensions of said end portion and said slot in said first electrode being such as to define a transmission line having an electrical length equal to one quarter wave length at the operating frequency and having a characteristic impedance Z1, the characteristic impedances being such that Zl Z0 and Z2 Z1, wherein Z0 is the characteristic impedance of the transmission line in which said switch is arranged.

3. A fiat strip transmission line switch comprising two fiat metallic conductors lying in spaced parallel planes, at least two switch electrodes each having two exterior surfaces, one of said switch electrodes having a slot in one end thereof providing two interior surfaces parallel to said two conductors, a right angle recess formed in each exterior surface of the other of said electrodes and extending parallel to said conductors to provide an end portion of reduced transverse dimension, means for selectively positioning said reduced end portion in said slot to provide a transmission line having an electrical length equal to one quarter Wave length at the operating frequency, a slot formed in said reduced end portion providing interior surfaces parallel to said metallic conductors, and having dielectric material positioned in and filling said slot, the electrical length of said slot in said reduced end portion being equal to one quarter wave length at the operating frequency.

4. The combination in accordance with claim 3, where in said transmission line formed between the interior surfaces of said slot in said one electrode and said reduced end portion of said other electrode has a characteristic impedance Z1, and wherein said slot in said reduced end portion has a characteristic impedance Z2, said characteristic impedances being such that Z1 Zo and Z2 Z1, wherein Z0 is the characteristic impedance of the transmission line in which said switch is connected.

5. The combination in accordance with claim 3, wherein said transmission line formed between the interior surfaces of said slot in said one electrode and said reduced end portion of said other electrode is open circuited at the both ends and wherein said slot in said reduced portion is short circuited at the end adjacent the open circuited end of said last mentioned transmission line and open circuited at the end adjacent the short circuited end of said last mentioned transmission line.

6. A fiat strip joint for conductors carrying radio signals comprising at least two switch electrodes lying in endto-end relation in a common plane, one of said switch electrodes having an end-slot in one end thereof, said slot having an electrical length equal to one quarter wave length at said predetermined frequency, the other of said switch electrodes having an end element, said end element having an electrical length equal to one quarter wave length at said predetermined frequency, an endslot in said end of said other switch electrode, said slot being filled with a dielectric material and having an elec trical length equal to one quarter Wave length at said predetermined frequency, said slot having a characteristic impedance Z2, means for selectively positioning said end of said other switch electrode in said slot of said one electrode, the relative dimensions of said slot and said end element being such as to define a transmission line having a characteristic impedance Z1, the characteristic impedances being such that Z1 Z0 and Z2 Z1, Wherein Z0 is the characteristic impedance of said transmission line.

References Cited in the file of this patent UNITED STATES PATENTS 

